1. Field of the Invention
This invention relates to an image forming apparatus.
2. Related Art
There have been extensively used image forming apparatuses of the type in which a latent image is formed on a surface of an image-carrying member, and a toner is caused to adhere to this latent image, thereby forming a toner image, and then this toner image is fixed to a recording medium.
In recent years, such an image forming apparatus has been advanced into a color version, and there has been an increasing demand for a high image quality and a high print speed. A variety of objects have now been printed, and therefore there has been a strong demand for the type of image forming apparatus capable of effecting high-speed printing on a continuous recording medium such as a paper roll or a plastic film.
In such an image forming apparatus, a color image is formed by superimposing toner images of different colors, that is, black (K), yellow (Y), magenta (M) and cyan (C), upon one another. For thus superimposing the toners of different colors on a continuous recording medium in high print speed, it is advantageous to use a so-called tandem-type apparatus in which a plurality of image forming units form toner images of different colors (K, Y, M and C), respectively, and these toner images are superimposed upon one another on the recording medium to thereby form a color image.
In the case of using the continuous recording medium, this recording medium is fed either by tractor paper feeder or by a roller paper feeder using rolls. In the roller paper feeder, the recording medium is grippingly held by rolls provided at front and rear portions of an image forming section, and a tension is applied to the recording medium by these rolls in order to stabilize the feeding operation.
In such an image forming apparatus, an elongation of the recording medium, produced by the applied tension, varies the speed of feed of the recording medium in the image forming section, so that misregistration of the superimposed toner images of different colors occurs in the direction of feed of the recording medium. The misregistration of the superimposed toner images adversely affects the image quality, and therefore some countermeasure must be taken.
However, the degree of elongation of the recording medium greatly varies from one material thereof to another. And besides, the tension, acting on the recording medium, varies during the operation of the apparatus, and therefore it is difficult to deal with this problem by beforehand adjusting the tension.
In order to deal with such problems, there has been proposed a method, as disclosed for example in JP-A-62-62764, in which registration marks are printed on a marginal portion of a recording medium, and the amount of misregistration is detected by reading this mark by the use of an optical sensor, and the printing position is corrected.
When the continuous recording medium is used in the conventional technique disclosed in the above JP-A-62-62764, those regions of this recording medium, on which the registration marks are printed, can not be used as an image forming region which is so originally intended, and this results in a problem that the recording medium is wasted. And besides, it is necessary to provide the high-precision optical sensor for reading the registration mark, and this is disadvantageous from the viewpoint of the cost.
It is an object of this invention to provide an image forming apparatus in which a position deviation of a recording medium can be detected highly precisely without the use of registration marks and a high-precision optical sensor.
This object can be achieved by an image forming apparatus according to the present invention, comprising:
a plurality of image forming units each comprising latent image-forming means for forming a latent image on an image-carrying member, developing means for causing a toner to adhere to the latent image, and transfer means for transferring the toner onto a continuous recording medium; and
recording medium feed means for feeding the recording medium;
the toner images, formed respectively by the plurality of image forming units, being transferred to the recording medium so as to form an image;
wherein a pair of recording medium feed speed detection means are provided upstream and downstream of the image transfer region, respectively, and there is provided image-forming position correction amount-outputting means for correcting a misregistration of the image recording position in the feed direction at the region in accordance with the speed detected by the recording medium feed speed detection means.
The latent image-forming means is preferable to include means for correcting a latent image-forming timing in accordance with a correction amount outputted from the correction amount-outputting means.
The latent image-forming means is preferable to include means for correcting a latent image-forming position in accordance with a correction amount outputted from the correction amount-outputting means.
The transfer means is preferable to include means for correcting a toner image-transferring position in accordance with a correction amount outputted from the correction amount-outputting means.